A variety of endless chain type trenchers have heretofore been utilized. Normally, such trenchers comprise an elongated frame having one end thereof pivotally mounted on a vehicle for movement in a vertical plane parallel to the path of travel of the vehicle. The digging frame provides a mounting for a power-driven endless chain carrying a plurality of spaced digging scoops. Thus the depth of the trench to be dug is determined by the angular position of the digging frame with respect to the horizontal, hereinafter called the digging frame angle. Such angular position can be determined by hydraulic cylinders appropriately connected between the digging frame and the vehicle. If the vehicle is moving over perfectly level ground, then it becomes a simple matter to control the depth of the trench being dug as a function of the angular position of the digging frame with respect to the horizontal; however, few if any trenching jobs involve perfectly level terrain and hence the angular position of the digging frame relative to the horizontal becomes an unreliable indicator of the trench depth as soon as the vehicle wheels or track encounter uneven terrain.
It has heretofore been proposed, for example in Studebaker U.S. Pat. No. 3,494,426, to control the working depth of an earth-working implement by attaching an upstanding mast to such implement and providing a plurality of vertically stacked sensors at the top of the said mast capable of detecting a reference plane defined by a rotating laser beam. If this type of control system were applied to a trencher, it would not result in an accurate control of the exact depth of the trench being dug by the endless chain because the lowermost digging point of such chain does not move vertically up or down but only moves arcuately about the pivotal mounting point of the trencher on the vehicle. More importantly, it is physically impossible to mount a vertically extending mast at exactly the lowest effective digging point of the endless chain. Instead, the mast must necessarily be mounted on a cantilevered sub-frame which has its forward end secured to the digging frame at an above ground level and its rearward end overlying the reaward portions of the digging chain. Thus, the resulting motions of the mast as the digging depth is varied, or when the vehicle wheels or track encounters an unevenness in the terrain, is an arcuate motion in a vertical plane about the pivotal mounting point of the digging frame on the vehicle.